1. Field of the Invention
The present invention relates generally to the field of surface inspection, and more particularly to simultaneously illuminating the surface inspected at multiple spots for enhanced anomaly detection.
2. Description of the Related Art
Conventional optical inspection methods employing scanning techniques typically illuminate a single spot on the surface inspected, where the inspection device scans the entire surface of the specimen for anomaly detection. In order to improve the signal-to-noise ratio associated with background scattering, previous designs have reduced the size of the illuminated spot. The result is an increase in the amount of time required to scan over the entire surface using the smaller spot. An increase in scan time is generally undesirable.
One way of addressing the SNR-spot size dichotomy is to employ a massively parallel inspection and imaging system that illuminates the specimen surface at a plurality of spots where scattered light from the spots are imaged onto corresponding detectors in a detector array. Such a massively parallel inspection system can enhance total inspection throughput, and may be further improved for enhanced performance in certain applications. Such a design is presented, for example, in U.S. Pat. No. 6,208,411. It may therefore be desirable to provide an improved multi-spot inspection and imaging system with enhanced characteristics.
With respect to multi-spot inspection and enhanced imaging, one design that improves the functionality of such a system is to employ a first objective for focusing an array of radiation beams to a surface and a second reflective or refractive objective having a large numerical aperture for collecting scattered radiation from the array of illuminated spots. The scattered radiation from each illuminated spot may be focused to a corresponding optical fiber channel so that information about a scattering may be conveyed to a corresponding detector in a remote detector array for processing. For patterned surface inspection, a cross- shaped filter may be rotated together with the surface to reduce the effects of diffraction resulting from a Manhattan geometry. Such a system may employ a spatial filter in the shape of an annular aperture to reduce scattering from patterns such as arrays on the surface. Different portions of the same objective may be used to focus the illumination beams onto the surface and simultaneously collect scattered radiation from the illuminated spots.
Another design that improves the functionality of a multi-spot inspection system uses a one-dimensional array of illumination beams directed at an oblique angle to the surface to illuminate a line of illuminated spots at an angle to the plane of incidence. Radiation scattered from the spots can then be collected along directions perpendicular to the line of spots or in a double dark field configuration, thereby providing enhanced performance and throughput. One such design having this improved simultaneous multi-spot inspection and imaging functionality is illustrated in U.S. Patent Publication 2004/0042001, inventors Mehdi Vaez-Iravani et al., filed Apr. 18, 2002 and published Mar. 4, 2004.
Such a multi-spot inspection system may provide obliquely incident illumination using, for example, individual incident spots focused onto the specimen surface as shown in FIG. 9 herein, but rather than spot centers lying on a line perpendicular to the optical axis, the incident optics are arranged such that the line connecting spot centers is at a 45 degree angle to the optical axis.
Implementation of this “45 degree angle design” can be highly challenging. From FIG. 9, spots 204 are in focus on the specimen surface, and thus the focal plane of the incidence objective is at approximately 45 degrees to the optical axis. Implementation of this orientation, including manufacturing and alignment of the components, can be particularly difficult, expensive, and potentially very time consuming.
It would be beneficial to provide a system for use in microscopy that overcomes the foregoing drawbacks present in previously known systems and provide an optical inspection system design having improved functionality over devices exhibiting those negative aspects described herein.